Booting Problems

So my parents are having issues with their PC, its well maintained and has lasted them about 4-5 years now the only parts changed from the original barebones kit is the GPU. Yet lately they have been having problems with booting up the system, one night they turned it off and the next day it wouldnt turn on. So my dad took apart the PC and put in a new power source. Once the power source was put in the PC booted up normally. My parents then turned it off for a short period of time (30 min I believe) and the PC still booted up fine. Yet once they turned it for for the night the next morning it wouldnt boot up. Could it be a virus of some sort? My dad keeps the PC clean, he has Avast running all the time, and almost every weeks does a full Malwarebytes scan. What could the issue be?